At present, for a backlight module of a liquid crystal display device, as illustrated in FIG. 1, a light guide plate (LGP) 01 and a bottom reflecting plate 02 are assembled in a manner of overlapping. In such structure, an air interlayer 03 having a thickness in an order of micron is formed between the LGP 01 and the bottom reflecting plate 02. The air has a refractivity n2=1 while the LGP 01 has a refractivity n1=1.5, thus light emitted by a backlight source 04 may be subjected to a total reflection when arriving at a contact interface between the air and the LGP 01, so as to be propagated inside the LGP 01. Moreover, the light will be emitted through a front surface of the LGP 01 upon subjecting to scattering at dot patterns 05 on the LGP 01, so as to change its direction of propagation. In the above solution where the LGP 01 and the bottom reflecting plate 02 are assembled in a manner of overlapping, a back plate is to be disposed below the bottom reflecting plate 02 for supporting, which, however, would make it difficult to achieve a slim design of the backlight module and the whole product.